They recommended putting the 6in. in the stock slot, the 3 inch forward of the door handle but not directly above it, and the tweet in the corner piece of the door right next to the window.
1st off, that Focal is a good set. Focal is a good brand in general, so if you hear something you like from them, go ahead and it. Same with Image Dynamics, Boston Acoustics, MB Quart, Rainbow, Arc Audio, DLS, Dynaudio, etc etc.
But, about where they want to put the 3" midrange I'm not too sure about. You'd actually want the 3" mids to be at a point where their pathlengths were equalized more.....which
isn't going to be anywhere high on the door. They would actually need to go lower (like, kickpanels) rather than higher to help get more even PLD (pathlength differences). Also, you wouldn't want them to be somewhere that you couldn't easily adjust their aiming. IMO, it would be better to go ahead and put that midbass in the sock locations, then put the midrange and tweeter in the kicks.
The midrange is the most important speaker in that set, and the midrange frequencies are where we get most of our imaging and staging clues from, as well as a majority of the tonality, etc. They are sensitive to being aimed poorly as well as PLD, and having them poorly set up will just kill the entire sound. You's really want to be able to aim them to maximize your soundstaging, as well as having their pathlengths close to even.